Percepts of biological motion disappear in slow-moving displays: Evidence for domain-specific agent perception

The most important stimuli we perceive may be other agents, given their direct effects on our fitness. Accordingly, perception may be specialized for processing agents, and one of the best-studied examples may be *biological motion*: displays of surprisingly few moving dots (‘point-light walkers’; PLWs) nevertheless give rise to rich percepts of locomoting agents — even when static frames from such displays appear as meaningless jumbles of dots. Does this reflect a distinct, domain-specific form of perception, or is it (merely) a complex instance of general motion/shape perception? Inspired by the fact that humans tend to move at certain minimum speeds, we explored this by simply asking how *slow* PLWs can be before percepts of biological motion are impaired or destroyed. Are those limits similar to lower-level motion perception thresholds? Or might biological motion have its own domain-specific lower “speed limit”? Observers viewed PLWs moving in place, embedded in noise (with extra irrelevant moving dots). Animations moved at typical speeds, or at speeds that were considerably slower but still very far above motion perception thresholds. And for slow displays, we always tested both duration-matched versions (with less overall motion) and trajectory-matched versions (that simply lasted longer). Across several experiments, observers tried to discriminate various properties — such as the direction of locomotion, the walker’s apparent gender, or even whether a PLW was present or absent in the first place. We always obtained the same results (which were also apparent as powerful phenomenological demonstrations): discrimination of each of these aspects of biological motion was greatly impaired (often to chance level) in the slower displays, despite the fact that the motion was still readily visible. This demonstrates the utility of exploring ‘slow visual cognition’, and supports the characterization of biological motion perception as a domain-specific form of visual processing.